3.11 yo, very restless sleeper - always tired. Any ideas?

Ds, 3.11 has been a very restless sleeper since about 1yo. He tosses and turns, gets up a lot, now that he's bigger has vivid nightmares. He also often seems distracted and sort of vacant during the day, in the same way I am when I'm really tired! I wondered whether he might have a physiological problem such as sleep apnoea (though he doesn't snore)? Is this possible in little children? I really want to help him to get restorative sleep in the night so he can enjoy his days more.